Euclid’s main claim to fame was attempting to axiomatize geometry. He started with self-evident geometrical truths, and using only those (and previously established proofs), proved other truths. Well sort of; he actually let some assumptions creep in. But he was successful enough to be regarded as the canonical example of this style of reasoning, and was a big influence on generations of philosophers, mathematicians and scientists.

In fact, you get a 3-for-1 by studying some Euclid. You get to see Geometry…